Name ___________________________________
Notes – Electrostatics
Electrostatics can be understood by remembering the structure of the atom:
protons
neutrons
electrons
Location
How tightly held?
Charge
Since protons are tightly bound in the center of the atom, they _________________ leave the atom.
Electrons are loosely bound, so the ________________ leave the atom.
Neutral Objects vs. Charged Objects
• A neutral atom has the same number of _____________ and ________________. Neutral
objects also have the same number of protons and electrons.
• The electrons contained within some objects are prone to _________________ other objects.
In fact, the process of an electron leaving one object to reside (perhaps only temporarily) in
another object is a common everyday occurrence.
• Electrons are often ___________________ and _________________________ an atom.
• Protons and neutrons are not removable by usual everyday methods.
• If something gets a negative charge, how did that happen? ____________________________
• If something gets a positive charge, how did that happen? _____________________________
Check your understanding: True or False?
1. An object that is positively charged contains all protons and no electrons. _____
2. A positively charged object is an object that has an excess of positive electrons. _____
3. A positively charged object has more protons than electrons. ______
4. Protons have a positive charge. _____
Charging Objects by Friction
When a rubber rod is rubbed against fur, electrons are removed from the ______ and deposited on the
_________. The rod becomes negatively charged because of an ________________ of electrons. The
fur becomes positively charged because of a __________________ of electrons.
When a glass rod is rubbed against silk, electrons are removed from the _____________ and deposited
on the ________________. The glass becomes _______________________ charged because of a
deficiency of electrons. The silk becomes _______________________ charged because of a excess of
electrons.
Check your understanding:
During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct
explanation for why the plastic strip becomes positively charged is that ...
a. the plastic strip acquired extra protons from the cotton.
b. the plastic strip acquired extra protons during the charging process.
c. protons were created as the result of the charging process.
d. the plastic strip lost electrons to the cotton during the charging process.
What is The First Law of Electrostatics? ________________________________________________
Name ___________________________________
Definitions:
________________________ are materials that permit electrons to flow freely from atom to atom and
molecule to molecule. (metals)
_______________________ are materials that impede the free flow of electrons from atom to atom
and molecule to molecule. (plastic, rubber, glass)
Copy the diagram, showing how a negative charge will spread throughout a conductor:
Check your understanding:
A conductor differs from an insulator in that a conductor ______.
a. has an excess of protons
b. has an excess of electrons
c. can become charged and an insulator cannot
d. has faster moving molecules
e. does not have any neutrons to get in the way of electron flow
f. none of these
Suppose that a metal sphere is charged positively by some method. The charge is initially deposited on
the left side of the sphere. Yet because the metal is a conductor, the charge spreads uniformly
throughout the surface of the sphere. The uniform distribution of charge is explained by the fact that _.
a. the charged atoms at the location of charge move throughout the surface of the sphere
b. the excess protons move from the location of positive charge to the rest of the sphere
c. excess electrons from the rest of the sphere are attracted towards the excess protons
What about neutral objects – are they attracted or repelled by anything?
Any ________________ object - whether positively charged or negatively charged - will have an
____________________ interaction with a neutral object. Positively charged objects and neutral
objects ____________ each other; and negatively charged objects and neutral objects _________
each other.
How can neutral objects be attracted to charged objects?
• Recall that electrons are loosely held in some atoms (especially ____________ atoms.)
• In conducting objects, electrons are so _______________ bound that they may be induced into
moving from one portion of the object to another portion of the object.
• To get an electron in a conducting object to get up and go, all that must be done is to place a
______________ object nearby the conducting object.
Copy the diagram showing how the electrons in the Coke can are induced into moving to one end:
The diagram you drew shows polarization of charges.
Name ___________________________________
_________________________ is the process of separating opposite charges within an object. The
positive charge becomes separated from the negative charge. By _________________ the movement
of electrons within an object, one side of the object is left with an excess of _______________ charge
and the other side of the object is left with an excess of _______________ charge.
The polarization process always involves the use of a _______________________ to induce electron
movement or electron rearrangement.
Charge polarization in insulators
But what if the object being polarized is an insulator? Electrons are not free to move across the surface
of an insulator. How can an insulator such as a wooden wall be polarized?
In a _____________________________ object, electrons are ____________________ into movement
from one side of the object to the opposite side.
In an insulator, electrons merely _______________________ themselves within the atom or molecules
nearest the outer surface of the object.
Copy the diagrams showing polarization within atoms:
Real life example:
Polarization is not charging! When an object becomes polarized, there is simply a redistribution of the
centers of positive and negative charges within the object.
Yet, there are still equal numbers of positive charges (_________________) and negative charges
(________________) within the object. While there is a separation of charge, there is ______ an
imbalance of charge. When neutral objects become polarized, they are still ______________ objects.
Charging by Induction
We’ve already seen that objects can be charged by _________ (rubbing different objects together.)
______________________ charging is a method used to charge an object without actually
touching the object to any other charged object.
Copy the diagram showing how two spheres can be charged to have opposite charges:
Name ___________________________________
Copy the diagram to show how one sphere can be charged by induction:
The Quantity of Charge:
The quantity of charge (q) can be defined in terms of the number of electrons, but the Coulomb (C)
is a better unit for later work. Copy the definition of a Coulomb here:
What is the charge of one electron? _________________________________
Practice problem: If 16 million electrons are removed from a neutral sphere, what is the charge on
the sphere in coulombs?
Coulumb’s Law
The force of attraction or repulsion between two point charges is _________________ proportional
to the product of the two charges and ________________________ proportional to the square of
the distance between them.
Copy the equation for Coulumb’s Law here:
Practice Problem:
What is the electrostatic force between two metal spheres, each having 5.0 C of charge? The
spheres are 0.30 m apart.